I would like to be able to use a USB port at the same time as an RS-232 port with my Arduino Uno board. I unwittingly bought a shield that (I think?) overrides the USB port when it is mounted to the Arduino board:


Would anyone be able to give me any advice on how to connect the hardware so that I can have two serial ports accessible from my Arduino Uno? The format I was thinking was (Arduino => RS-232 Sheild):

GND => GND 5V => 5V Digital Pin 2 => 232RXD Digital Pin 3 => 232TXD

The accompanying code for initializing the serial port which I've written:

const int rxpin = 2; // pin used to receive
const int txpin = 3; // pin used to send
SoftwareSerial Serial_rs232(rxpin, txpin); // new serial port

Any ideas as to why this may or may not be a good method of using the hardware I have available for the purpose I had in mind?


GND => GND 5V => 5V Digital Pin 2 => 232RXD Digital Pin 3 => 232TXD

A better choice of board would be the Leonardo, since you could connect such a shield and use the USB port right out of the box. However, since you have the Uno, there is a solution.

From Arduino:

The SoftwareSerial library has been developed to allow serial communication on other digital pins of the Arduino, using software to replicate the functionality (hence the name "SoftwareSerial"). It is possible to have multiple software serial ports with speeds up to 115200 bps. A parameter enables inverted signaling for devices which require that protocol.

It then goes on to reccomend the library AltSoftSerial.

Here is some example code from the link above:

#include <AltSoftSerial.h>

AltSoftSerial altSerial;

void setup() {
  Serial.println("AltSoftSerial Test Begin");
  altSerial.println("Hello World");

void loop() {
  char c;

  if (Serial.available()) {
    c = Serial.read();
  if (altSerial.available()) {
    c = altSerial.read();

It acts like a standard serial port, only you declare the connection as a variable first. This code is written for a board with an additional serial port, like you have.

All is good, except for one problem:

The problem is the shield is wired to connect to pins 0 and 1: the ones used by the USB adapter. The AltSoftSerial library uses pins 8 for TX and 9 for RX. (Note: You cannot use PWM on pin 10 on the Uno... it should still work as a digital pin.) In that case you would not attach the shield by plugging it into the Arduino, and you would wire it like this:

GND => GND | 5V => 5V | Digital Pin 8 => 232RXD pin 0 on the shield headers | Digital Pin 9 => 232TXD 1 on the shield headers

So then, you would be manually wiring the shield to the Arduino with jumpers. As long as it doesn't connect to any other Arduino pins I'm not aware of (it shouldn't besides maybe 13 for a LED or something like that), it should function like it would stacked and using pins 0 and 1.

Note: (Obviously) You cannot connect anything to the headers on the shield (You cannot connect to pin 5 and get a signal).

  • Thanks for the informative reply! Do you definitely advocate AltSoftSerial in place of the SoftwareSerial library then? Your hardware recommendation involving the Uno sounds great, but I'm unsure about why you've chosen the RX/TX digital pins on the shield, in favour of the pins labelled RS-232 on the wiki diagram?
    – user_name
    Mar 20 '14 at 21:43
  • @user_name I do think AltSS. would be the best choice: Arduino themselves said this library is better for simultaneous connections (see link in my post). For the second part, that should work as long as you don't stack them on top of each other. The reason is, both the USB adapter and the RS-232 use pins 0 and 1. By doing the wiring I said, you are "rerouting" them onto different pins on the Arduino. Just, most people don't have female jumpers needed to connect to the "232TX"/"232RX"... Remember use pins 8 and 9 and connect the TX and RX accordingly. Mar 20 '14 at 23:12

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